OHAP. vm] OIL AND PETROL ENGINES

247

the petrol, having so low a vaporizing point, at once turns
Into vapour and forms with the air an explosive mixture.

The screw adjustment—or needle valve as it is called__allows

the richness of the mixture to be adjusted. Pig. 79 shows a
"better and more familiar way of doing the same thing. Air is
sucked in past the nozzle of the jet F and out by the opening
K. In rushing past the jet it sucks up a petrol spray, which
evaporates as it mixes with the air. On the left of the figure

^PRESSURE INLET

FIG. 80.—Lanchester Surface Carburettor.

is seen the float chamber for keeping the petrol level constant.

It operates much as does a ball and cock feed to a water

cistern. B is a needle valve which gets pushed down on to

its seating by the levers D when the float C rises to the top.

This stops more petrol coining in until the petrol-level sinks

so much as to let the float down till the levers open the needle

. valve again, when more petrol flows in. The weight of» the

\ float is so adjusted that the petrol-level is kept at just the

I right height. It is the custom to have the petrol standing

just below the top of the jet, but it works even if standing

I!